In recent years the design and operation of fluid cracking operations with an adjacent catalyst regeneration system has gone through some unusual design transitions with a view to improving the efficiency of the combination operation as well as the product distribution obtained from such an operation. In particular, the designs have been concerned with utilizing fluid crystalline aluminosilicate cracking catalysts in volume to oil feed ratios which minimize the catalyst inventory of the operation, improve product selectivity, and improve the recovery of available heat generated in the catalyst regeneration system. Catalyst regeneration has been improved by increasing the catalyst bed regeneration temperature by the recycle of hot regenerated catalyst thereto and by particularly promoting the combustion of CO to CO.sub.2 therein by thermal and catalytic effects. That is, some recent design changes incorporate recycling of hot regenerated catalyst for admixture with cooler spent catalyst recovered from the hydrocarbon conversion operation such that the combined temperature of the mixed catalyst streams is sufficently high to rapidly initiate coke burning and accomplish catalytic CO (carbon monoxide) burning in a substantial portion of a dense fluid bed of catalyst being regenerated. It has been found in some regeneration operations that the CO concentration is the flue gas exceeds emission standards and unburned residual carbon on regenerated catalyst becomes undesirably high; that is, above about 0.05%. Several design parameters and apparatus arrangements have been proposed to solve this problem. However, these designs often suffer from a number of problems such as high catalyst inventory, low temperature, incompletely regenerated catalyst, a lack of operating flexibility to control catalyst recycle or employ external apparatus configurations or arrangements in an effort to effect more suitable control in the operation, thereby contributing to operating costs.
On the other hand, some regenerator vessel designs and arrangements have been substantially increased in height, thereby increasing construction costs. In these arrangements, the circulating catalyst inventory and necessary catalyst bed hold-ups have increased and high temperature metallurgy requirements have increased. These factors contribute to increased material, maintenance and operating costs of the units.
The present invention is concerned with yet another design and operating arrangement for improving operating flexibility and reducing the catalyst inventory of the regeneration system. More significant, however, is the arrangement of apparatus for reducing the regenerator apparatus size for a given oil feed through-put without a loss of operating flexibility. Other advantages of the improved apparatus arrangement of this invention and method of operating will be more apparent from the following discussion.